What is SNR?

First question I have is what is SNR? also
My line was recently swapped due to a fault, my SNR is 15 is this normal?

Signal to Noise ratio. I believe its how much "louder" your signal is to the background noise on your line.

I THINK its the higher the number, the better

Edited by wrtpeeps (Sun 01-Oct-06 16:42:03)

SNR is 'signal to noise ratio' and is different to the 'noise margin' quoted by ADSL modems and routers.
The 15db you're seeing is a 'noise margin', and may be viewed as being the difference between the ADSL signal, and the background noise on the line.
In very rough terms, the background noise could increase by 15db before your router will lose the ADSL signal and get swamped by noise.

15db is a very high noise margin figure to have (the default is 6db for BT wholesale provided lines), and only occurs when the BT monitoring systems have decided your line is unstable at the old noise margin.
This can occur as a result of a line fault (in my case, a dodgy router and a series of thunderstorms). When the BT systems 'bump' up the noise margin, usually is steps of 3db, until they see a stable line.

Now you have your fault repaired, you're stuck with a 15db noise margin, which will be giving you a much lower sync (connection) speed.
You now face a battle with your ISP and BT wholesale (who you can't talk to - even though it may help), to get your margin reduced, or your line re-trained.
Best of luck! I've been at this stage for about three months, and have got nowhere (other than frustrated, and now holding the belief that BT is truly one of the most incompetent companies out there).

Edit: To clarify 'SNR'. It is a ratio between signal level and noise level (both being expressed in db). As it's a ratio, it has no unit.

Edited by adebov (Sun 01-Oct-06 16:59:13)

apologies, it appears i gave you the totally wrong info, thats adebov for the information

Or it could be that he's not on MAX.

But with limited info, and 15db margin being the common result of a dodgy line, it seemed the most logical suggestion.

I am getting the 15db reading from the DMT tool could I not reduce the Target Noise Margin in DMT or should I ask BT to retrain the line?

Sorry I didn

Info from home Hub


Uptime: 0 days, 0:12:12
Modulation: G.992.1 Annex A
Bandwidth (Up/Down) [kbps/kbps]: 448 / 7,072
Data Transferred (Sent/Received) [KB/KB]: 131.00 / 407.00
Output Power (Up/Down) [dBm]: 12.0 / 19.5
Line Attenuation (Up/Down) [dB]: 10.5 / 23.0
SN Margin (Up/Down) [dB]: 24.0 / 15.5
Vendor ID (Local/Remote): TMMB / TSTC
Loss of Framing (Local/Remote): 0 / 0
Loss of Signal (Local/Remote): 0 / 0
Loss of Power (Local/Remote): 0 / 0
Loss of Link (Remote): 0
Error Seconds (Local/Remote): 0 / 0
FEC Errors (Up/Down): 0 / 91
CRC Errors (Up/Down): 0 / 0
HEC Errors (Up/Down): 0 / 0

What are the FEC Errors?

As you're so close to the maximum 8128kbps, you may be able to get there by tweaking the target margin using DMT (assuming the home hub will accept it).
Most margin tweaks only allow a maximum shift of around 5db, but lowering to 10db target should give you enough of a speed boost to get to the magical 8128kbps (you may not get all the way, as it appears you've got "interleaving" turned on).
Interleaving assists the router with error correction, when a line is a bit flaky or noisy. This is where the FEC errors come into it. They are corrected errors and nothing much to worry about.

Whether to get your ISP to turn off interleaving, or not, is down to how you use the internet. As you'll probably only get an extra couple of hundred kbps by having it turned off, unless you do a lot of online gaming (where ping times are important), you may as well leave alone.
As I said, it aids error correction in your router, and although it results in a slightly lower sync (meaning, if you sort out your margin, you won't be able to sync at 8128kbps with interleaving), giving you a 7Mbps maximum speed profile instead of 7.15Mbps. Hardly anything worth worrying about, and you'll still get a maximum speed of around 6700kbps (of course, it's highly unlikely you'll get that speed at all times, or maybe never).

If you do manage to tweak your router, you may have to keep an eye on it. AFAIK only the Speedtouch routers can save the tweaked settings, other tweakable routers return to pre-tweaked settings following a reboot or power cycle.
Trust me, it's easier to tweak the router yourself, than to get BT to reduce the margin at their end (because that will NEVER happen).

I got my line changed because I was going through a high level complaint, I need to speak to them again because my bras is only 2500 currently so I will ask to have my line retrained and interleaving turned off now I am on the new line, I just dropped the Target noise to minimum (I didn

OK, you're now only 32kbps away from maximum sync. This may well be caused by interleaving being 'on'.
Your current sync will put you on a 7Mbps BRAS profile, within 3-4 days (how long it takes to catch up with increased sync speeds).
If you manage to get BT to understand what you mean by "please turn off and disable interleaving on my line", you may well get the maximum 8128kbps (which will give you a BRAS profile of 7.15Mbps).

It's up to you if you want to go through the hassle of getting your line retrained (or, at the very least, having your target noise margin lowered to 6db), neither of which may ever happen (despite what your ISP says).
The only thing you'll gain by having interleaving turned off, and your line retrained, is a theoretical extra speed of 150kbps (about 2% speed boost), and the knowledge you don't have to keep checking your sync speed and noise margin (in case your router should reboot and lose the changes).

If you do approach your ISP, and manage to get your line retrained (so you can still get full speed, with your router set back to "default" target), let us know (as you'll be one of the first to get it done manually).

There's also talk of an automatic process which gradually lowers your target margin, but this seems to take blocks of 14 days continuous sync (without any drop outs), and requires you to resync after the 14 days, to take advantage.
I've only ever seen one post, by one user, who states this has happened.

The main reason I am going to press for the retrain is if I put my phone number into the wholesale website it tells me I can only get a 1meg connection and 1.5 on max, now I am not sure if the retraining will sort that out but I am worried if I don

In reply to:

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The main reason I am going to press for the retrain is if I put my phone number into the wholesale website it tells me I can only get a 1meg connection and 1.5 on max

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I guess something is very wrong if you get that kind of estimate with an attenuation of only 23 dB!

Until the line was repaired last week I was getting 43

Retrain will not fix the 1.5Mbps estimate.
This is a fault in the BT wholesale database which, unless I'm mistaken, has no physical (or software) link to the ADSL line training process.
I have no idea how to get it fixed, although I think it's important to fix it, as the poor speed estimate may prevent you from migrating to certain ISPs that take not of it (e.g. AOL), and may prevent you from going LLU, to Bulldog, or BE, etc. (as LLU providers use the estimate to decide whether to offer you high speed services).

I'm in the same position. The BT checker says I can get 512kbps fixed speed (even though I had 1Mbps before upgrading to max), and a maximum speed of 1.5Mbps on ADSLmax (even though I'm currently sync'd at 2944kbps, and could sync around 4000kbps before this 15db nonsense started).
Although my line will support 3-4Mbps, I cannot get AOL, Bulldog, BE, or any other LLU, because the BT checker says my line is only able to support 1.5Mbps.

Anyone else reading this have the slightest idea how to get BT to correct the database, with actual measured values (rather than estimated ones, based on postcode or line length)?
Another thing, I don't believe my reported line length of 6.94km. This seems far too long for a 57db attenuation, and far too long for me to be getting 3-4Mbps with a noise margin of 6db. How to get this corrected, also?

When I fist signed up I was getting about 5.5 meg about a month ago something happened and I started getting 2 meg with a bras of 100k, at that point the checker speed went to 1.5 meg and minimum fault speed had gone down to 1.2 meg I was told that this could be caused by the training on the line saying that

Edited by miller75 (Sun 01-Oct-06 20:49:42)

As your postcode gives you a higher speed estimate, it looks like your data is based on measured values (from the faulty line). This really needs updating with good data (best of luck).
Although you can get your line running at the highest speed, by tweaking your own router, the main reason for wanting a retrain, is to correct the stupidly low fault threshold. In your case, caused by a faulty line, in my case caused by rubbish internal wiring.

If you can convince your ISP (BT retail, I believe) to push the appropriate requests to BT wholesale, go for it. You've very little to lose!

I will speak to them and let you know if I have any success

In reply to:

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Edit: To clarify 'SNR'. It is a ratio between signal level and noise level (both being expressed in db). As it's a ratio, it has no unit.

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It's important to stress that SNR measured in dB is a power measurement, and is not linear. So a 3dB increase is double the power, and a 6dB increase is 4x the original power.

In reply to:

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Edit: To clarify 'SNR'. It is a ratio between signal level and noise level (both being expressed in db). As it's a ratio, it has no unit.

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Sorry, but this is nonsense. Signal-to-noise ratio is measured in decibels (dB). Signal power levels are measured in watts.

The dB is defined as 10 x log(10) of the ratio P1/P2, where in this case P1 is the signal power and P2 is the noise power. In other words the decibel is a logarithmic unit for specifying a ratio (NOT a power level).

Eric

Actually, SNR can be a straight ratio (with no units, as dividing units into each other, removes the unit), or can be expressed in db.
As you say, SNR (with db unit) is 10 x log(10) of the power ratio, but the power ratio expressed without units, and prior to multiplying by 10log(10), is still valid.
Quite what units you use when comparing power level in dbm (which is a valid measurement BTW - dbm is the ratio of the signal level, compared to 1mW) - Maybe still db (as the true dbm/dbm ratio - before mucking about multiplying by 10log(10) - has no units).

All of us are made to suffer pay up and smile whatever service we get. BT believe we should just accept whatever we may be lucky to receive if almost anything.
Instead of just putting up with all this effort and time we should act like the french for example and deliver the message of the paying punters and take it to the world outside. How long is all the knowledge and the friends who just ask for the service that we should be receiving put up with this.
Its time for all to think of some legitamate action. Its time to take this matter to your politicians and make them deliver some results, it can be done. It should be done.
All we want is what we are paying for. Does this have some support please.

You were correct,
BT will not retrain the line or to put it another way they will not fix a fault on equipment owned by BT that I rent from them. After waiting over 2 weeks for an answer to specific question put to them I have given up and asked them to close my complaint (I know it shouldn